Earthquake Reaearch in China  2018, Vol. 32 Issue (3): 301-314
The Key Technical Points and Main Characteristics of the New National Standard of Magnitude
Liu Ruifeng, Wang Liyan
Institute of Geophysics, China Earthquake Administration, Beijing 100081, China
Abstract: The New Magnitude National Standard of General Rules for Earthquake Magnitude (GB17740-2017) is the state mandatory standard. It was released on May 12, 2017, by the General Administration of Quality Supervision, Inspection and Quarantine of the People's Republic of China and the Standardization Administration of the People's Republic of China. This paper introduces the necessity of revising the national standard of magnitude, and the main contents, technical points and primary features of the new national standard of magnitude, so that it can be applied better in practice.
Key words: Local magnitude ML     Surface wave magnitude MS     Body wave magnitude mb     Moment magnitude MW     National standard

INTRODUCTION

Magnitude is a measure of the size of an earthquake and is one of the basic parameters of an earthquake. It is usually represented by the letter M (Chen Yuntai et al., 2004). There are four commonly used magnitude scales: local magnitude ML, body wave magnitude mb, surface wave magnitude MS and moment magnitude MW.

In 1935, Richter introduced the local magnitude ML (Richter C.F., 1935) in the study of the earthquake in Southern California, which is now called Richter magnitude, abbreviated as Richter scale. Although the Richter scale is entirely empirical, the method of measurement is simple and easy to use. What is more important is to lay the foundation for the development of magnitude. Although the local magnitude ML is useful, it can't be used to determine the magnitude of global far-range earthquakes because of the type of seismograph used and the applicable epicentral distance range. In 1945, Gutenberg B extended the method of measuring local magnitude ML to become teleseismic (Gutenberg, 1945b, 1945c). The amplitude of the surface wave is the largest in the record of the shallow-sourced far-field earthquake. For an earthquake with an epicentral distance of Δ>2, 000km, the period of the maximum value of the surface-wave horizontal amplitude is generally about 20s. The surface wave with a period of about 20s corresponds to the Airy phase on the dispersion curve of the surface wave train, and Goutenberg (1945a) proposed the surface wave magnitude scale. For deep-sourced earthquakes, surface waves are not developed, but P waves are clearly phased at far-range distances, so Gutenberg et al. (1956) used body waves (P, PP, S) to determine magnitudes. This is called body wave magnitude (Gutenberg, 1945b, 1945c). Magnitude saturation exists in the local magnitude, surface wave magnitude, and body wave magnitude. In order to overcome the magnitude saturation of large earthquakes, Kanamori (Kanamori, 1977), Purcaru and Berckhemer (Purcaru et al., 1978) and Hanks and Kanamori (Hans et al., 1979) almost simultaneously proposed the moment magnitude scale MW in 1977-1979.

Magnitude measurement in China began with the measurement of local magnitude ML. The local magnitude calculation formula proposed by Richter in 1935 is only applicable to the California region of the United States, and the observation instrument used was the Wood-Anderson torsion seismograph, which has obvious limitations. In the mid-1950s, the Chinese seismic station was not equipped with the Wood-Anderson torque seismograph that was used to establish the Richter local magnitude ML standard. Richter's proposed magnitude scale could not be copied intact. In view of this, based on the attenuation characteristics of seismic waves in North China, and combined with the instrumental characteristics of short-period seismographs and medium and long-period seismographs used in China at the time, Mr.Li Shanbang gave the Chinese local magnitude ML gauge function and calculation formula. After January 1966, the China Earthquake Report was adopted by Guo Lücan et al. (1981), which was proposed and used, in 1966 but was not officially published until 1981. The surface wave magnitude was measured using two horizontal levels of surface wave data MS based on the Beijing Seismic Station. In 1988, Chen Peishan et al. (1988) proposed the method of selecting the maximum amplitude and period of the vertical Rayleigh surface wave for the determination of MS7, making MS7 measured by the China Earthquake Networks Center consistent with the surface wave magnitude MS measured by the National Earthquake Information Center (NEIC) of the US Geological Survey(USGS). In body wave magnitude determination, China has used the body wave magnitude measurement method proposed by Gutenberg and Richter to determine the short-period body wave magnitude mb and medium-long periodic body wave magnitude mb using the maximum velocity of the P or PP wave vertical particle motion.

In order to regulate the measurement of earthquake magnitudes and social applications, under the leadership of Academician Xu Shaoxie in 1999, experts from the Institute of Geophysics, China Earthquake Administration and other units completed preparation of the national standard General Rules for Earthquake Magnitude (GB17740-1999). It was first released on April 26, 1999 (Xu Shaoxie et al., 1999; Xu Shaoxie, 1999). In view of the differences in the method of magnitude determination at the time among the International Seismological Center (ISC), National Earthquake Information Center (NEIC) of the US Geological Survey (USGS), the China Earthquake Administration (CEA), the Japan Meteorological Agency (JMA) and other international seismic agencies, no consensus has yet been reached. Experts have repeatedly demonstrated that China should continue to maintain its own magnitude measurement system. The standard stipulates the method of measuring the surface wave magnitude MS using two horizontal data and defines the surface wave magnitude MS as the earthquake magnitude M released to the public for use in seismic news reports, earthquake emergency response, and earthquake prediction publication, propaganda and earthquake applications for earthquake and disaster mitigation. General Rules for Earthquake Magnitude (GB17740-1999) has been implemented since 1999 and has played an important role in earthquake monitoring, earthquake prediction, earthquake prevention and disaster reduction and news reporting in China, and has achieved good scientific and social benefits.

After more than ten years of development, China's seismic observation system has achieved a historic breakthrough in "digitalization and networking". By the end of 2007, seismic stations that were officially operating were all built into digital stations, with instrument characteristics, data transmission methods, data analysis, treatment methods and the timeliness of magnitude measurement having undergone a fundamental change. Since the Implementation of the Magnitude National Standard (GB17740-1999), China has accumulated a large amount of seismic observation data, and has gained new understanding in earthquake magnitude measurement. Significant progress has been made in the method and release rules, which have been gradually applied. In 2012, a working group was set up by the Department of Earthquake Monitoring and Prediction of China Earthquake Administration to initiate the revision of General Rules for Earthquake Magnitude (GB17740-1999). Under the guidance of Academician Chen Yuntai and Academician Xu Shaoxie, Institute of Geophysics of China Earthquake Administration, China Earthquake Networks Center, and National Marine Environment Forecasting Center of China organized scientific and technical personnel to carry out relevant research and standard preparation work. After research and demonstration of more than 20 topics, and the consultation of the Magnitude Group of the International Association of Seismology and Intra-Earth Physics (IASPEI), the International Seismological Center (ISC) and other international agencies and world-renowned seismologists, and the solicitation of the opinions of relevant national ministries, universities and scientific research institutions, the regulation was reviewed by the leaders of China Earthquake Administration, Science and Technology Commission and National Seismological Standardization Technical Committee. It was completed four years later in January 2016. On May 12, 2017, the General Administration of Quality Supervision, Inspection and Quarantine of the People's Republic of China and the National Standardization Administration released the People's Republic of China National Standards Announcement No.11 (2017), approving 334 national standards, including 6 national mandatory standards, and 328 national recommendations. General Rules for Earthquake Magnitude (GB17740-2017) is one of the national mandatory standards.

1 NECESSITY OF REVISION OF MAGNITUDE NATIONAL STANDARDS

According to Technical Specification for Seismic and Precursor Digital Observations (Seismic Observation) (China Earthquake Administration, 2001) set forth in General Rules for Earthquake Magnitude (GB17740-1999), Chinese seismological stations should start to measure the local magnitude ML, short-period body wave magnitude mb, medium and long-period body wave magnitude mB, surface wave magnitude MS and surface wave magnitude MS7 in daily seismic monitoring before December 1, 2017. The China Seismological Network uses the surface wave MS as an external M seismicity when an earthquake is reported, and the provincial seismic network measure the local ML, and use the empirical formula (Table 1) to convert ML into a surface wave MS during the earthquake reporting period.

Table 1 Transformation of the local magnitude ML and the surface wave magnitude MS

At present, the problems existing in the China Earthquake Network's magnitude determination and release are mainly manifested in the following aspects.

(1) There are differences in the methods of measuring surface wave magnitude from major international earthquake agencies. The China Earthquake Networks Center does not use the magnitude method recommended by the International Society of Seismology and Intra-Earth Physics (IASPEI) in 1967 in daily seismic analysis and seismic rapid reporting. The surface wave magnitude measured in China is larger of 0.2 than that of major international earthquake agencies (Xu Shaoxie, 1999). For example, the April 20, 2013 Lushan earthquake, Sichuan, the China Earthquake Networks Center released a surface wave MS of 7.0, and the National Earthquake Information Center (NEIC) measured a surface wave MS of 6.8 and a moment magnitude MW of 6.6, and the released moment magnitude was MW6.6. The difference of the two released magnitudes is 0.4. According to the National Earthquake Emergency Preparedness Plan revised on August 28, 2012, if the released magnitude reaches 7.0, the State Council, PRC will initiate a first-level response to earthquake relief. However, considering the actual post-quake intensity distribution, the IX-degree area was only 208km2, and the released magnitude was obviously too high. Similarly, there were such problems with the Yutian earthquake in Xinjiang on February 12, 2014, the Ludian earthquake in Yunnan on August 3, 2014, and the Jinggu earthquake in Yunnan on October 7, 2014. The phenomenon of China posting a significantly higher magnitude has increasingly attracted public attention.

(2) The magnitude of the moment magnitude MW is not measured in the daily work of the seismic network. Moment magnitude is a mechanical quantity that describes the absolute size of an earthquake, and is also the most ideal physical quantity for measuring the magnitude of an earthquake. The international earthquake science community recommends that the moment magnitude be the preferred magnitude scale (USGS, 2002). According to the requirements of General Rules for Earthquake Magnitude (GB17740-1999), the China Seismic Network releases an earthquake magnitude M, which is actually a surface wave magnitude MS. Due to the difference in the magnitude distribution rules between the China Seismic Network and major international seismic agencies, the magnitude of the externally released earthquakes is quite different. For example, on August 8, 2017, for the Jiuzhaigou earthquake, the China Earthquake Networks Center released an external surface wave MS7.0, and the National Earthquake Information Center (NEIC) released a moment magnitude MW of 6.5, with a difference of 0.5. Therefore, the China Seismological Bureau recommends that the State Council initiate a secondary response to earthquake relief. In addition, due to the saturation of surface wave magnitude, the surface wave magnitude MS is smaller than the moment magnitude MW by 0.2-0.4 for earthquakes above 8.5. For example, on March 11, 2011, for the earthquake in the sea area of northeast Japan, due to the saturation of the surface wave magnitude, the China Earthquake Networks Center released a surface wave magnitude of MS8.6 to the public, but the Japan Meteorological Agency (JMA) and NEIC released a magnitude of MW9.0 and the surface wave magnitude was 0.4 smaller than the moment magnitude.

(3) Magnitude deviation can be caused by the transition between different magnitudes. According to the requirements of General Rules for Earthquake Magnitude (GB17740-1999), the National Seismic Network should use "released magnitude" M externally, so there is a magnitude-shift relationship between the actual measured magnitude and the released magnitude of the provincial seismic network, making a large deviation between the provincial speeding earthquake release and the National Seismic Network release. For example, for the aftershocks of the Minxian-Zhangxian mainshock in Gansu Province on July 22, 2013, the Gansu Digital Seismic Network determined the local magnitude ML5.6, converted it to MS5.3 according to Table 1, and released it as M5.3. The National Seismic Network determined the surface wave magnitude MS5.6 and released a magnitude of M5.6. The difference was 0.3. Seismic activity statistical forecasting methods in earthquake prediction generally use released magnitudes obtained from the determination of magnitude in accordance with empirical formulas, which increases the statistical error and is not conducive to the research and application of statistical forecasting methods.

General Rules for Earthquake Magnitude (GB17740-2017) mainly includes three parts: magnitude measurement methods, release rules, and usage rules.

(1) Measurement method. The new standard specifies six types of measurement: local magnitude ML, short period body wave magnitude mb, wide band body wave magnitude mB(BB), surface wave magnitude MS, broadband surface wave magnitude MS(BB) and moment magnitude MW. The physical meanings indicated by different magnitudes are different. Therefore, the new standard stipulates that the measured magnitudes should not be converted to each other.

(2) Release rules. The seismic network directly uses different magnitude scales to release externally based on the magnitude of the earthquake. The new standard stipulates that the moment magnitude MW will be used as the focus of the seismic network to determine the magnitude and be the preferred magnitude for external publication. For earthquakes that cannot measure moment magnitude in time, the new standard stipulates that for shallow source earthquakes with ML < 4.5, the local magnitude ML should be selected as the externally released magnitude. For shallow source earthquakes with ML≥4.5, the broadband surface wave magnitude should be selected. MS(BB) is the magnitude released to the outside world. For medium-to-deep source earthquakes, the body-wave magnitude is selected to release magnitude to the outside world. The seismic network should ensure that earthquake parameters are quickly released to the community in the first instance to meet actual needs such as earthquake emergency and news reports.

(3) Usage rules. The new standard clearly stipulates the social applications related to the magnitude of earthquakes such as earthquake information release, scientific popularization, news reporting, earthquake prevention and disaster reduction, and requires the use of the released magnitude M.

3 TECHNICAL POINTS

Table 2 shows the comparison of the new magnitude national standard (GB17740-2017) and the original standard (GB17740-1999) plus the original observation technical specifications on the magnitude determination method. As can be seen from Table 2, GB17740-1999, the original observation specification requires that when measuring the magnitude, the broadband digital seismic recording is simulated in the recordings of the traditional DD-1 short-period instrument, SK long-period instrument and 763 long-period instrument, which is actually a continuation of the analog record magnitude measurement method and does not fully exploit the advantages of broadband digital seismic recording. This is mainly based on the following considerations: ① To maintain the continuity of the magnitude measurement. ② To continue to use the original gauge function to determine the local magnitude, surface wave magnitude and body wave magnitude. After a certain period of time for the data to be accumulated in the broadband recording, a new method for measuring magnitude based on broadband digital seismic recording was studied.

Table 2 Comparison of GB17740-2017 and GB17740-1999 plus techniques of magnitude measurements in the former observation technical specification

It can also be seen from Table 2 that in the new standard, the local magnitude ML, short-period body wave magnitude mb, and surface wave magnitude MS are consistent with the original observation specifications in order to maintain the continuity of the magnitude determination, while the broadband with the body wave magnitude mB(BB), broadband surface wave magnitude MS(BB) and moment magnitude MW are measured on the original broadband recording to take advantage of the broadband digital seismic recording.

3.1 Moment Magnitude

Moment magnitude is the magnitude that the new standard is required to focus on. Both the China Earthquake Networks Center and the Provincial Digital Seismic Network Center must measure the moment magnitude MW. The moment magnitude MW should be obtained by substituting the measured seismic moment M0 into the standard form formula of M0 recommended by IASPEI (2013) in units of N·m (Bormann, 2011).

 ${M_{\rm{W}}} = \frac{2}{3}\left({\lg {M_0} - 9.1} \right)$ (1)

In the formula, M0 is the seismic moment and the unit is N·m.

In particular, it should be noted that equation (1) should be used to calculate moment magnitude instead of using the following formula to calculate moment magnitude.

 ${M_{\rm{W}}} = \frac{2}{3}\lg {M_0} - 6.1$ (2)

At first glance, equation (2) and equation (1) are equivalent, and using equation (2) to calculate the moment magnitude should yield the same result as equation (1), which is not exactly the case. Equation (1) is equivalent to

 ${M_{\rm{W}}} = \frac{2}{3}\lg {M_0} - 6.06$ (3)

When the magnitude of the moment is accurate to 0.1, due to rounding errors, when using equation (2) to calculate moment magnitude, it sometimes leads to a difference of 0.1.Therefore, IASPEI (2013) specifies that the moment magnitude should be calculated using the equation (1) instead of using the constant term ($\frac{2}{3}$ ×9.1) on the right side of equation (1) to calculate and round equation (2) in advance.

In particular, the provisions of IASPEI (2013) differ from those of the US Geological Survey (USGS, 2002) in two points: ① The unit of M0 in equation (1) uses dyn·cm but not N·m(1 dyn·cm=10-7N·m); and ② USGS (2002) uses a formula that is equivalent to equation (2)

 ${M_{\rm{W}}} = \frac{2}{3}\lg {M_0} - 10.7$ (4)

to calculate MW, not a formula equivalent to equations (1) and (3)

 ${M_{\rm{W}}} = \frac{2}{3}\left({\lg {M_0} - 16.1} \right)$ (5)

or

 ${M_{\rm{W}}} = \frac{2}{3}\lg {M_0} - 10.73$ (6)

to calculate MW.

In GB17740-2017, mB(BB) and MS(BB) are wide-band body wave and wide-band surface wave magnitudes respectively. The body wave period used is 0.2s < T < 30.0s, and the surface wave period is 3s < T < 60s. The establishment of two broadband magnitude scales is mainly based on the following considerations: ① The use of digital seismic instruments has the characteristics of wide frequency band and large dynamics. ② Well-coupled with conventional body-wave magnitudes and surface-wave magnitudes, and with earthquakes in China the medium-long period body wave magnitude mb and surface wave magnitude MS measured by the network are close to each other, and the range of the measured magnitude is wider. ③ It is directly measured on the original velocity type broadband recording, which is convenient for computer automatic measurement and is suitable for earthquake rapid reporting.

In 1996, Liu Ruifeng et al. (1996) proposed a calculation method for the determination of surface wave magnitude by using flat-rate digital seismic data, based on the characteristics of broadband digital seismic recording.

 ${M_{\rm{S}}} = \lg \left( {\frac{{{V_{\max }}}}{{2{\rm{ \mathsf{ π} }}}}} \right) + 1.66\lg \mathit{\Delta } + {\rm{3}}.{\rm{3}}$ (7)

In the formula, Vmax is the maximum value of the vertical movement speed of the surface wave. Using equation (7) to determine the magnitude method is simple. Using the Global Seismograph Network (GSN) data, we measured the surface wave magnitudes of the MS6.1 earthquake in the eastern waters of Taiwan, China on September 28, 1992, the MS6.3 earthquake in Ruoqiang, Xinjiang on October 2, 1993, the MS6.7 earthquake in the Taiwan Strait, China on September 16, 1994, and the MS6.7 earthquake in Yangbajin, Tibet on March 20, 1993. The results were compared with the NEIC measurements in the United States. The average magnitude deviation was 0.05.

In daily work, the Chinese National Seismic Station must determine the medium and long-period body wave magnitude mb. Through a comparative study of 1985-2004 records, it is found that the mid- and long-period body wave magnitude mb measured in China reaches a saturation level above 8.0. Since the time required for the determination of long-period body wave magnitude mB is short, the magnitude of the larger earthquake can be quickly determined. This is the advantage of our determination of mB (Liu Ruifeng et al., 2005; Bormann et al., 2007). Liu Ruifeng et al. proposed a broadband body wave magnitude measurement method for mB(BB) based on the characteristics of broadband digital seismic recording (Bormann et al., 2009; Liu Ruifeng et al., 2015).

 ${m_{{\rm{B}}\left({{\rm{BB}}} \right)}} = \lg \left({\frac{{{V_{\max }}}}{{2{\rm{ \mathit{ π} }}}}} \right) + Q\left({\mathit{\Delta }, h} \right)$ (8)

In the formula, Vmax is the maximum value of P-wave column particle velocity, and Q(Δ, h) is the gauge function of vertical P-wave body wave magnitude.

Wide-band surface wave MS(BB) and broadband body-wave mB(BB) are both based on China National Seismic Network wide-band digital seismic data and have been established by the International Association of Seismology and Earth Physics (IASPEI). According to the Working Group on Magnitude Measurements, MS(BB) and mB(BB) are two new scales in the IASPEI's new magnitude criteria (Bormann et al., 2009).

3.3 Local Magnitude

The measurement method and calculation formula for the local magnitude ML are unchanged, but the zoning gauge function is required. The gauge function of local magnitude is closely related to the attenuation characteristics of seismic waves. The geological structure in China is complex and the regional difference of seismic attenuation is obvious. It is obviously unreasonable to use the same gauge function in different regions. The project team used 1973-2002 seismological network observation data from various provincial-level seismic stations in China, and used 105, 282 active earthquakes recorded at 1, 308 stations with a total of 375, 744 seismic data sets. The data were obtained from the Northeast China, North China, South China, Southwest China, the Qinghai-Tibetan region and Xinjiang area. The gauge functions were R11(Δ), R12(Δ), R13(Δ), R14(Δ) and R15(Δ) (Wang Liyan et al., 2016). The distribution of ML≥1.0 earthquakes in Chinese provinces from January 1973 to December 2002 is shown in Fig. 1, and the distribution of corresponding seismic stations is shown in Fig. 2.

 Fig. 1 Distributions of ML ≥ 0.1 earthquakes (circles) in China from January 1973 to December 2002

 Fig. 2 Distributions of seismic stations (triangles) in China from January 1973 to December 2002

The applicable areas of China's five largest regional scale functions are as follows: ① Gauge function R11(Δ) in the Northeast and North China: Heilongjiang, Jilin, Liaoning, Inner Mongolia, Beijing, Tianjin, Hebei, Shanxi, Shandong, Henan, Ningxia and Shaanxi. ② South China gauge function R12(Δ): Fujian, Guangdong, Guangxi, Hainan, Jiangsu, Shanghai, Zhejiang, Jiangxi, Hunan, Hubei, Anhui. ③ Southwestern gauge function R13(Δ): Yunnan, Sichuan, Chongqing, Guizhou. ④ The Qinghai-Tibetan area gauge function R14(Δ): Qinghai, Tibet, Gansu. ⑤ Xinjiang gauge function R15(Δ): Xinjiang.

3.4 Body Wave Magnitude

The body wave magnitudes measured in the new standard include the short period body wave magnitude mb and the broadband body wave magnitude mB(BB). When measuring mb and mB(BB), only P-wave serials (including P, pP, sP, and even PcP and its wake, generally taken before PP waves) are used instead of PP and S waves. There is no limit to the time tmax for measuring the maximum amplitude of the P-wave trajectory, and the only requirement is to be able to determine the maximum velocity of its particle motion or the maximum displacement of particle motion.

When measuring mB(BB), the speed is used directly without simulation. The results of the study (Bormann et al., 2009) show that for earthquakes with M≥6.0, the mB(BB) and MW are not significantly different. Generally, the measurement of mB(BB) takes only 1-2 minutes, while the determination of MW requires about 20 minutes. The German Geoscience Research Center (GFZ) assisted in the construction of the Indonesian Earthquake and Tsunami Warning Network. The mB(BB) was used for earthquake and tsunami warning and achieved good results.

3.5 Different Magnitudes Will Not Be Converted

The different magnitude scales use different periods and different wave serials, and different periods and different wave columns carry different complex source information. Local magnitude ML, short-period body wave magnitude mb, broadband body wave magnitude mB(BB), surface wave magnitude MS, and wide-band surface wave magnitude MS(BB) reflect the magnitude of radiated seismic wave energy in different periods of the seismic wave. The dominant period of local magnitude ML is about 0.8s, the dominant period of short-period body wave magnitude mb is about 1.0s, the dominant period of surface wave magnitude MS is about 20s, and the broad band surface wave magnitude MS(BB) and broadband body wave magnitude mB(BB) fully exploit the characteristics of broadband digital seismic data, and the applicable range of surface wave and body wave periods are significantly increased. The period range of MS(BB) is 3.0s-60.0s, and the cycle range of mB(BB) is 0.2s-30.0s. Therefore, using different magnitude scales can more objectively represent the size of the earthquake, that is, except for the magnitude of the moment, not all of the above magnitudes can objectively represent the magnitude of all earthquakes, which is the variation of the magnitude.

Because of the different meanings represented by different magnitudes, in actual seismic monitoring work, uniform vibrations between different magnitudes and broadband body-wave shocks are not allowed to be converted. The provincial seismograph network no longer uses empirical formulas to convert local-magnitude ML into surface-wave magnitude MS to provide seismic information to the outside when they perform earthquake quick reporting.

3.6 Determined Magnitude and Released Magnitude

The guiding ideology for formulating new national standards for magnitude is "the difference between inside and outside".

Internally, due to the complexity of the earthquake process, the complexity of the seismic wave propagation medium, and the complexity of the seismic base, a single magnitude scale cannot be used to quantitatively describe the size of all earthquakes. The most direct method is to use different magnitude scales to describe the basic characteristics of different earthquakes. Different magnitude scales actually describe the size of the earthquake from different angles and show the variety of magnitudes. Therefore, in the daily work of the seismic network, the measurable local magnitude ML, surface wave magnitude MS, broadband surface wave magnitude MS(BB), short-period body wave magnitude mb, broadband body wave magnitude mB(BB) and moment magnitude MW must be determined. These six kinds of magnitudes require that the seismic network actually measure the magnitude.

Externally speaking, from the perspective of social applications, the seismic network must promptly release a single, non-perplexing earthquake magnitude to government agencies and the general public. This is a manifestation of the social function of earthquake monitoring. In other words, an earthquake has only one magnitude for government agencies and the general public.

Therefore, from the perspective of practical application, the magnitude is divided into "determined magnitude" and "released magnitude". The measured magnitude is actually measured by the seismic network and should have subscripts such as: local magnitude ML, short period body wave magnitude mb, broadband body wave magnitude mB(BB), surface wave magnitude MS, wideband surface wave magnitude MS(BB) and moment magnitude MW. The released magnitude is selected from the measured magnitudes. There is no subscripted mark, which is denoted by M. It is only used for earthquake-related social applications such as earthquake information release, news reports, scientific popularization, and earthquake emergencies.

3.7 Magnitude Release

GB17740-2017 provides the following rules for magnitude release.

(1) When the seismic network releases the seismic quick report information, it should give priority to selecting the magnitude MW as the externally released magnitude for earthquakes that can determine the seismic moment M0 in a timely manner.

(2) When the seismic network releases the seismic quick report information, it is necessary to determine the magnitude of the earthquake to be released to the outside according to the following principles for earthquakes that cannot determine the seismic moment in time M0: ① For shallow earthquakes with ML < 4.5, select the local magnitude ML, for externally released magnitudes. ②For shallow source earthquakes with ML≥4.5, the broadband surface wave magnitude MS(BB) should be selected as the externally released magnitude. ③ For short-period body-wave magnitudes for medium-source and deep-sourced earthquakes, the mb or broadband body wave magnitude mB(BB) is the externally released magnitude.

(3) In preparing the earthquake catalog, the seismic network should list all measured magnitudes and externally released magnitude M at the same time. For earthquakes whose earthquake moment M0 can't be determined in time, ML and MS(BB) are respectively released with a limit of M4.5 earthquakes, based on the following.

The project team used different magnitudes of 44, 523 earthquakes measured by the China Seismic Network from 1983 to 2004, and used orthogonal regression methods to obtain the relationship between different magnitudes (Bormann et al, 2007). The results show that for different earthquakes, using different magnitude scales can more objectively describe the size of the earthquake, which is the variation of the magnitude.

(1) When M < 4.5, there is little difference among various magnitudes, and local magnitude ML can be used to indicate the magnitude of earthquakes below M4.5. This is consistent with the results obtained by Gutenberg(1945a, 1945b, 1945c) in 1945.

(2) When 4.5≤M < 8.0, the broadband surface wave magnitude MS(BB) and moment magnitude MW are the closest, especially for earthquakes with M≥6.5, and the difference between MS(BB) and MW is within 0.1. Therefore, when 4.5≤M < 8.0, if a reliable moment magnitude MW has not been obtained, the magnitude of the earthquake can be represented by the wide-band surface wave magnitude MS(BB).

(3) When M≥8.0, the surface wave magnitude appears saturated, and the moment magnitude MW can represent the size of the M8.0 earthquake.

(4) For mid-deep and artificial earthquakes (including nuclear explosions) with focal depths greater than 60km, body-wave magnitudes can be used to represent the size of the earthquake.

The project team selected the 531 seismic broadband digital seismic data recorded by the National Seismic Network, including 14, 000 seismic records. The magnitude range was 3.8≤MS≤8.7. Basic amplitude seismic waves data, corresponding periods, epicenter distances, and azimuths of stations of the M8.0 including the Chinese traditional magnitudes of mb, mB, MS7 and MS and IASPEI new magnitudes of mb, mB(BB), MS, of each earthquake have been man-determined and the eight kinds of magnitude values above have been measured (Liu Ruifeng et al., 2015)

One of the important results obtained through comparison is that the fitting formula between the broadband surface wave MS(BB) and the moment magnitude MW is obtained by using the orthogonal regression method.

 ${M_{{\rm{S}}\left({{\rm{BB}}} \right)}} = 1.34{M_{\rm{W}}} - 2.19\;\;\;\;\;\;\;\;\;{M_{\rm{W}}}<6.8$ (9)
 ${M_{{\rm{S}}\left({{\rm{BB}}} \right)}} = 1.04{M_{\rm{W}}} - 0.39\;\;\;\;\;\;\;\;\;{M_{\rm{W}}} \ge 6.8$ (10)

The MW and MS(BB) are calculated from equation (9) and equation (10), and see Table 3 for the value list. As can be seen from Table 3, MS(BB)and MW deviations are within 0.1 for earthquakes with M≥6.5. According to the current seismic system data output, it takes at least 20 minutes to obtain a reliable MW, and the MS(BB) can be automatically determined within 1 min of the computer. This is the advantage of measuring the MS(BB).

Table 3 Comparison of MW and MS(BB)
4 MAIN FEATURES

Compared with GB17740-1999, GB17740-2017 has the following features.

(1) The measurement method is more scientific. GB17740-2017 fully reflects the magnitude and complexity of the magnitudes and specifies six methods for measuring magnitudes. The moment magnitude MW is used as the focus of the seismological network to determine the magnitude, and a Chinese magnitude determination system is preliminarily established to be more scientific in the magnitude determination.

(2) The measurement result is more accurate. GB17740-2017 fully draws domestic and foreign research results, and uses the characteristics of broadband digital seismic recording to not only make the measured magnitude more accurately represent the magnitude of seismic energy, but also eliminate the systematic deviation of the previous magnitude formula. Due to the introduction of regional scale functions such as moment magnitude, broadband body-wave magnitude, wide-band surface-wave magnitude, and regional magnitude, the implementation of the new standard will make the seismic measurements of M4.5 or higher more consistent with major international seismic agencies. It will enable earthquakes below M4.5 to more fully reflect regional characteristics.

(3) Measurement speed is faster. In GB17740-2017, wide-band surface-wave magnitude MS(BB) and wide-band body-wave magnitude mB(BB) are measured directly on the original wide-band recording in the vertical direction, which is convenient for computer automatic processing; the magnitude conversion and waveform simulation are cancelled. This improves the timeliness of earthquake quick reporting and emergency response.

(4) The release rules are more reasonable. In GB17740-2017, the statistically dependent magnitude shifting is no longer used, but based on the seismic characteristics, the results that best reflect the actual situation of the earthquake are selected from the measured magnitude scales to be released first. This not only ensures the scientific properties of the magnitude, but also takes into account the actual needs of society.

5 CONCLUSION

Earthquake prevention and disaster reduction is a social welfare undertaking of the Chinese Government. It is a public service that the earthquake authorities at all levels provides to the society and is a priority for the People's Republic of China. The revision of the national standard, General Rules for Earthquake Magnitude, is the requirement of the China Earthquake Administration to perform earthquake monitoring and forecasting duties in accordance with the Earthquake Prevention and Mitigation Law. It is also an important approach and measure to further strengthen the social management of earthquake prevention and disaster reduction and improve public service capabilities. The rapid and accurate determination of the magnitude of a destructive earthquake is of critical importance to the earthquake emergency and assistance after the earthquake; the standardization of earthquake magnitude determination and social applications will be used to promote earthquake prevention and forecasting, earthquake disaster prevention, earthquake emergency rescue, and other earthquake prevention and reduction work. Earthquake science research is of great significance.

The newly revised magnitude national standard fully considers the continuity of magnitude determination, fully embodies the characteristics of broadband digital seismic recording, and introduces the latest results of seismic observation techniques at home and abroad, reflecting the diversity of magnitude and achieving a connection with the new scale of magnitude of IASPEI and the major earthquake agencies around the world in terms of magnitude measurement. The method of magnitude measurement is more scientific, measurement results are more accurate, measurement speed is faster, and the release rules are more reasonable, so that the measured magnitude can more accurately represent the size of the earthquake itself.Especially for earthquakes of M4.5 and above, the magnitude released by China Earthquake Networks Center is consistent with the major international earthquake agencies, and there is no systematic bias.

This paper has been published in Chinese in the journal of Seismological and Geomagnetic Observation and Research, Volume 39, Number 2, 2018.

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